The art comprises various types of illumination systems of the type where a continuous high power light source, e.g. an Hg or Xe lamp, illuminates a plurality of illumination points on a light-sensitive medium via a given type of light modulators.
However, the technology has not been a great commercial success, because many light valve types have a very low coefficient of utilization, and, accordingly, distribution of light over a large illumination face will typically give greater optical losses and thereby cause the emitted optical power on the illumination point to be reduced considerably. As a result of this circumstance, the available optical energy will frequently tend to be concentrated in one specific small illumination area, rather than trying to distribute it over a large area for an extended period of time because of the limited illumination power and thereby achieve a reduced illumination power over the individual illumination points.
A drawback of this prior art caused by the above-mentioned problems is that it is necessary to place a very large number of light valves in a light valve array on a very small area, as it is very difficult to distribute sufficient optical power over a large area, and also difficult to achieve a uniform surface illumination.
Computer to plate technology, which is known e.g. from U.S. Pat. No. 5,049,901 in which e.g. printing plates are illuminated via DMD light valves, involves the problem that it is not possible to have sufficient optical power distributed over a large area. The patent thus describes how illumination on a scanning line is maintained as best as possible for the longest possible period of time by illuminating the same scanning line with several rows of light valves. Another consequence of the relatively low illumination power may also be that special printing plates having an increased light sensitivity have to be used, which for one thing are expensive in use and for another make greater requirements with respect to storage and use than the conventional printing plates. A further possible consequence of this relatively low achievable optical energy is that the illumination time of the system must be increased considerably. This increase in time consumption, however, is not very expedient as the overall necessary exposure time for a printing plate is increased considerably.
A further drawback of optical distribution over a large area is that the use of e.g. a large number of light sources may give rise to rather pronounced edge problems which occur at the boundary areas between the illumination profiles and the illumination area of the individual light sources. These edge problems have previously been avoided either by illuminating an illumination area with the same light emitter, or alternatively by illuminating each individual illumination point with a separate optical fibre. Systems of the first-mentioned type, which are known from U.S. Pat. No. 4,675,702, have the drawback that the illumination area is restricted physically, thereby calling for a complicated relative mechanical movement between the illumination unit and the substrate.
The last-mentioned type provides a uniform illumination on the illumination face, as the illumination intensity varies between each illumination point so that the variation of the illumination intensity is not visible. A drawback of the last-mentioned type, which is known from U.S. Pat. No. 4,899,222, is that the system is extremely complicated, as an optical fibre is required for each illumination point. This means that the light distribution from the light source to the light modulators requires the use of a very large number of optical fibres, and that a very precise adjustment of each individual optical fibre is required with respect to the light source as well as the light modulators. It should be recalled in this connection that each individual optical fibre must be re-adjusted by routine exchange of light source.
The above-mentioned light modulator systems are additionally vitiated by the drawback that the transmission attenuation is very high, whereby high power illumination on the medium to be illuminated is extremely difficult or downright impossible.